He commanded a Burgundian flotilla organized by Philip III the Good, Duke of Burgundy (r. 1419–1467) in May 1441 at the request from the Knights Hospitaller to help defend Rhodes against the Mamluk Sultan of Egypt.
When a second Mamluk fleet assailed the island, the Burgundians helped the Hospitallers lift the siege of the town of Rhodes which lasted for forty days.
[1] Next, in response to the Byzantine appeal for help against the Ottoman advance, Philip instructed de Thoisy to lead his squadron to reinforce a squadron of four galleys built or hired at Nice and Venice, commanded by Waleran de Wavrin, to join Pope Eugene IV's planned Crusade.
De Wavrin's four galleys sailed in July 1444 but failed to stop Murad II's Anatolian army from crossing the Bosporus.
Although the Emperor of Trebizond warned him that the people of Georgia were Christians, de Thoisy went ahead with his campaign, claiming that his orders were to fight all schismatics who did not obey the Pope.